The effect of the Na/K ratio on the phase transition temperatures of CuCl nanocrystals in potassium-alumina-borate glass is studied. The size effect in the location of the exciton absorption is confirmed. By increasing the content of sodium ions in glass, it is possible to increase the melting point of CuCl crystals with a mean size of 3.1 nm from 152 to 168 °C. It is assumed that in the matrix of potassium alumina-borate glass, instead of pure CuCl crystals, a CuCl-RCl (R = Na, K) solid solution crystallizes during the heat treatment.